The present case relates in particular to such X-ray detectors and their use which comprise a layer converting X-rays (quanta) into a plurality of charge carrier pairs, what are known as directly converting X-ray detectors. Charge carriers of the charge carrier pairs (typically under the effect of an electrical field) thus namely form a charge pulse, and if the X-ray detector comprises a plurality of picture elements (which correspond to pixels of an X-ray image to be produced), then as a rule such charge pulses strike a single picture element. The present case involves a deviation from this rule: if the individual X-ray quanta comprise energy levels above what is known as the k-edge (for example 27 keV for Cd, 32 keV for Te), then k-fluorescence dominates in the case of the photo effect. A k-fluorescence photon is re-emitted in addition to the photoelectron, and this has roughly the energy of the k-edge, or, more precisely, the difference in the binding energy levels of the k-shell and the shell from which a succeeding electron originates. Owing to the not inconsiderable mean free path length of the k-fluorescence photon of about 100 μm or 60 μm respectively for Cd or Te respectively, in addition to the two cases where the k-fluorescence photon is reabsorbed by the same picture element as the primary photon or it completely leaves the detector material, the case may accordingly also occur where in a neighboring picture element it is reabsorbed to the picture element reabsorbing the primary photon.
The case of reabsorption in a neighboring picture element, which is substantially more probable than the k-fluorescence photon not being absorbed at all in the detector material, accordingly leads to two events being detected and the energy of the primary quantum being divided over the two picture elements.
If there is a desire to detect the energy levels of the X-ray quanta and count them, then the occurrence of the k-fluorescence photon is disruptive.
There is consequently a desire to detect the coincidence of two charge pulses on adjacent picture elements in order to be able to correct the count values or the detected energy levels. There is therefore firstly an attempt in accordance with a predefined criterion to detect whether two charge pulses occur simultaneously on adjacent picture elements anyway. However, it should accordingly also be ensured that this involves a true coincidence, i.e. the same event, and not, for instance, a false coincidence, i.e. the random simultaneous striking of two primary X-ray quanta in adjacent picture elements.
Two different approaches have previously been taken in this regard:
In the case of a conventional approach discussed first an analog coincidence circuit is provided (before the analog-digital conversion) which connects each picture element to its directly neighboring picture elements and, in the case of two events being simultaneously detected in accordance with a predefined criterion, is capable of totaling the energy of the two events in an analog manner before this is converted to digital. The advantage of this is that the energy is correctly output (is restored), and if true coincidence is a prerequisite, the correct number of X-ray quanta is also counted. The drawback of the analog coincidence circuits lies in that these are highly complex, in that namely highly integrated silicon technology is required for their implementation. They also consume a lot of energy, so cooling of the X-ray detector is a challenge.
Previously discussed in the field as a second approach is the idea of using a digital coincidence circuit (i.e. following analog-digital conversion), wherein this merely has a veto function: if two adjacent picture elements respond then one of the two count values is disregarded. The advantage lies in the fact that the circuit is significantly simpler and in that the correct number of quanta is counted if true coincidence is assumed. The drawback lies in that the energy of the absorbed quantum cannot be reconstructed (restored) since the energy of the quantum from one of the two picture elements is disregarded.